A charge pump is an electrical circuit that can take in a direct current (“DC”) voltage and generate an output voltage that is higher than the original. An alternate configuration is a negative charge pump which generates a voltage that can be below ground.
A prior art embedded dynamic random access (“eDRAM”) memory cell is illustrated in FIG. 1. During a write to this memory cell, a high voltage is put on the ‘Gate’ and the voltage on the ‘Node’ 11 gets stored by the capacitor 13. The higher the voltage, the faster the capacitor will be charged. A charge pump can be used to generate this high voltage.
During a read of the memory cell, a high voltage is put on the ‘Gate’ 15 and the voltage that is stored on the capacitor 13 can be read at the ‘Node’ 11. The higher the voltage, the faster the read of the memory cell.
During standby, the gate voltage will be driven low to turn off the N-Type transistor 17. Leakage thru this transistor 17 will drain the capacitor. A charge pump can be used to generate this negative voltage to minimize the leakage.
With reference to FIGS. 2-4, in a typical positive charge pump, the positive charge pump will create a new voltage that is higher than the power supply (called VPP). A comparison is usually done to figure out whether the output voltage is high enough. The compare is usually made between some reference voltage and a divided down output voltage.
If the output voltage is too low, the pump can be activated. Looking now at FIG. 2, we see P-type 19 and N-type 21 transistors which act as digital switches in FIGS. 3 and 4. A shorted connection refers to the transistor switch being closed while an open connection refers to the transistor switch being open. There are two phases of operation of the charge pump, which are charging and pumping. During charging shown in FIG. 3, the power supply voltage VDD appears across the capacitor 23. During pumping, the charge built up across the capacitor 23 can be discharged into the output VPP. Together with the comparison and reference voltage these components may make up a charge pump system.
As noted above, voltage generators can be used to create DC voltages that are higher than the input power supply. When the final DC voltage needs to be above two times the power supply, a cascaded design is usually the primary solution. One or more voltage generators may be used to create intermediate voltages, which are then used in the final stage to create the output voltage.
For example, FIGS. 5-8 illustrate a typical two stage prior art voltage generator and its various states. In such, the charge capacitor 25 doubles the power supply (VDD), and then pumps. As a result, the voltage across the capacitor 25 reaches VDouble.